Methods of soldering electronic parts include the soldering iron method, the flow soldering method, the reflow soldering method, and the like.
The reflow soldering method is a method in which a solder paste comprising a solder powder and a flux is applied by printing or dispensing only to necessary locations of a printed circuit board, on which electronic parts are then placed, and the solder paste is melted by a heating apparatus such as a reflow furnace to solder the electronic parts to the printed circuit board. The reflow soldering method can perform soldering with excellent productivity and reliability in that not only can a large number of locations be soldered in a single operation but also in that there is no occurrence of bridge formation even in an electronic part having a narrow pitch and solder does not adhere to unneeded locations.
Solder paste conventionally employed in the reflow method uses a Pb—Sn alloy to form a solder powder. The Pb—Sn alloy is advantageous in that its melting point is 183° C. for a eutectic composition (Pb-63Sn), so it has little thermal effect on electronic parts which have poor resistance to heat, and in that it has excellent solderability, so there is little occurrence of soldering defects such as unsoldered portions or dewetting. When electronic equipment which is soldered with a solder paste using this Pb—Sn alloy becomes old or malfunctions, instead of being upgraded or repaired, it has been discarded. When printed circuit boards are discarded, they have been disposed of by burial rather than incineration. This is because solder is metallically adhered to copper foil conductor of printed circuit boards and the copper foil and the solder cannot be separated for reuse. When acid rain contacts printed circuit boards which have been disposed of by burial, Pb in the solder leaches out and pollutes underground water. If underground water containing Pb is drunk for long periods by humans or livestock, there is a concern of the occurrence of Pb poisoning. Therefore, there has come to be a strong demand in the electronics equipment industry for so-called “lead-free solder” which does not contain Pb.
Lead-free solders have Sn as a main constituent element. Presently-used lead-free solders are binary alloys such as Sn-3.5Ag (melting point: 221° C.), Sn-0.7Cu (melting point: 227° C.), Sn-9Zn (melting point: 199° C.), and Sn-58Bi (melting point: 139° C.), as well as these solders to which an additional element such as Ag, Cu, Zn, Bi, In, Sb, Ni, Cr, Co, Fe, Mn, P, Ge, or Ga is suitably added. In the present invention, “based alloy” means to include the alloy itself as well as alloys in which at least one additional element is added to the binary alloy. For example, an Sn—Zn based alloy includes an Sn—Zn alloy and alloys in which at least one additional element is added to Sn—Zn, and an Sn—Ag based alloy includes an Sn—Ag alloy as well as alloys in which at least one additional element is added to Sn—Ag.
The Sn—Ag based, Sn—Cu based, and Sn—Ag—Cu based lead-free solders which are mainly used at present have a melting point of at least 220° C. Therefore, when they are formed into a solder paste and used in the reflow method, the peak temperature at the time of reflow ends up being at least 250° C., and they had the problem that electronic parts or printed circuit boards underwent thermal damage during reflow heating.
An Sn—Zn based lead-free solder has a melting point close to that of a conventional Pb—Sn eutectic solder. For example, an Sn-9Zn eutectic lead-free solder has a melting point of 199° C. Therefore, this type of solder makes it possible to use the reflow profile for a conventional Pb—Sn eutectic solder as it is. As a result, the thermal effect on electronic parts or printed circuit boards is minimized. However, an Sn-9Zn eutectic solder paste has poor wettability, so an Sn-8Zn-3Bi lead-free solder which contains Bi added to an Sn—Zn alloy having a nearly eutectic composition is frequently used. Sn—Zn based lead-free solders have superior properties compared to other lead-free solders in that they have a melting point close to that of conventional Sn—Pb solders. In addition, Zn contained therein is an essential elements for humans, so compared to other lead-free solders, they are not harmful to the human body, and there are large reserves of Zn compared to In, Ag, Bi, and the like, so its unit cost is low. Accordingly, in spite of the fact that their solderability is not so good, Sn—Zn based lead-free solders are used as solders for solder paste, and in particular, they are used for printed circuit boards for which Sn—Ag based lead-free solder cannot be used because the parts do not have sufficient heat resistance.
However, Sn—Zn based lead-free solders have the problem that after soldering of a printed circuit board having Cu lands such as commonly used FR-4 printed circuit boards, the strength of the resulting soldered joint decreases if the printed circuit board is left at a high temperature. This is because reactivity between Zn and Cu is high. Therefore, when an Sn—Zn based lead-free solder is used for soldering of a printed circuit board with Cu lands, if a high temperature state continues for a long period, the Zn in the solder migrates through the solder alloy layer and enters into the Cu lands, and a large number of voids called Kirkendall voids develop between the resulting intermetallic compounds and the solder. These voids decrease the bonding is strength of solder and worsen the reliability of soldering. Therefore, Au plating is necessary when using an Sn—Zn based lead-free solder, resulting in the problem of an increase in the manufacturing costs of electronic equipment.
One parameter causing a decrease in bonding strength when soldering Cu lands of a printed circuit board using an Sn—Zn based lead-free solder is humidity. If humidity is high, Zn is readily oxidized into Zn2+ ions, thus causing Zn in an Sn—Zn based lead-free solder to readily migrate through the solder alloy layer and enter into Cu and leading to the formation of a large number of voids. This phenomenon becomes marked when the humidity is at least 80% even when the temperature is 100° C. or below. Voids also easily develop in conditions where moisture condensation occurs, thereby causing a decrease in the bonding strength of solder.
Means which have been disclosed for increasing the bonding strength of these Sn—Zn based lead-free solders include a solder paste comprising an Sn—Zn based lead-free solder mixed with a flux in which a metal powder containing a Group 1B metal as a constituent element is dispersed (JP 2002-224880 A1) and a lead-free solder alloy having Ag added to an Sn—Zn based lead-free solder (JP H09-253882 A1).
Solder alloys containing nanoparticles have been proposed in the art. Nanoparticles are powder having a particle diameter on the nanometer order. They can enter the spaces between particles of usual solder powder having a particle diameter on the micrometer order and exhibit various properties. Examples of technology using nanoparticles which have been disclosed include a solder having increased resistance to fracture by disposing Ni nanoparticles on the surface of spherical particles of solder (JP 2003-062687 A1) and a solder alloy of an Sn—Zn based lead-free solder in the form of a self-organized nanoparticles (JP 2004-268065 A1).
Patent Document 1: JP 2002-224880 A1
Patent Document 2: JP H09-253882 A1
Patent Document 3: JP 2003-062687 A1
Patent Document 4: JP 2004-268065 A1